The present invention relates generally to high-strength steels and, more particularly, to a method for increasing the impact toughness of aluminum-killed steels as well as microalloyed steels, with or without aluminum additions. Still more particularly, the invention relates to a method of processing these classes of high-strength steels containing grain-refining additions to prevent particle embrittlement therein.
The deleterious effects of second-phase particles on the toughness of high-strength steels have received a great deal of attention in the art over the past 30 years. This attention has primarily focused on particle embrittlement induced by non-metallic inclusions, aluminum nitride precipitates and large alloy carbides retained through the processing of the steel. More recently, microalloying technology has been employed in the production of grain-refined, 0.1%-0.4% carbon steels that are hardened and then tempered at temperatures below the range associated with the onset of tempered martensite embrittlement. The applicability of this technology has been, however, somewhat limited from the standpoint of restricted carbonitride solubility at carbon contents above 0.2%. A review of the literature suggests that particle embrittlement, which is enhanced by limited precipitate solubility, may have a significant effect on the development of toughness in this class of high-strength steels. The embrittlement may be alleviated via austenitization at high temperatures, but the decreases in precipitate content that alleviate the embrittlement also provide a necessary and sufficient condition for austenite grain growth, thereby defeating the original purpose of the microalloying technology. Considering the potential for second-phase particles to degrade the toughness of tempered martensitic micro-structures, very little work has been done in either defining the extent of embrittlement induced by microalloy carbonitrides or developing heat treatments to minimize the effects of particle embrittlement.
The present invention addresses the aspect of particle embrittlement and defines a thermal/thermomechanical process to provide a fine austenite grain size while avoiding or eliminating the effects of particle embrittlement in high-strength steels containing grain-refining additions.
The method of the invention is easily incorporated into a mill processing scheme for the production of annealed machining bars and with only minor modifications to existing production lines. In addition, the process of the invention is suitable for treating quenched and tempered tubes and is most useful in the production of heat-treated forgings.
The present invention provides a method for increasing the impact toughness and grain coarsening resistance of killed steels containing grain-refining elements, particularly the class of steels utilizing aluminum in conjunction with various microalloying elements such as Ti, Nb, and V, either singly or in combination.